These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fridge Horror: The reason why Miyu's father seems so out of it is because Miyu's mother gave him her vampire blood. Now imagine growing up with a father who stumbles around life as if in a waking dream.
Hearing Miyu's soft, gentle, and so terribly creepy giggles in the OAV is known to send people into fits of terror. Hear them around 7:16.
Ho Yay: In the TV series, pretty blatant between Larva and Garline, especially from the latter's side: he acts like a spurned lover, mentions Larva's "beauty", takes his hand, caresses his cheek... Considering that Garline is an expy of Lemures, it's not really a surprise.
Les Yay: In the manga, the difference between how Reiha treats Ranka and Miyu is very... extreme. When Ranka is injured in the process of sheilding Miyu from her wrath, Reiha becomes very courtly and kisses her wounded hand. Ranka's reaction to this is a shudder.
In the anime, Lemures crosses it with gusto. Not only he kidnaps Larva and seals him away, but he uses a very cruel trap to recruit the Shinma armor. He tricks a desperate young widow into giving him her life to bring her husband back, but once she does so and dies, the hubby's soul is bound to an armor in his Big Fancy House, making him his memory-less and Ax-Crazymook.
Then we have Chisato crossing it in the TV series, when she murders Hisae and Yukari as soon as her Shinma powers awaken. She has kind of an excuse in the case of Hisae since there's some degree of More Than Mind Control from Shidon, but in the case of Yukari she does it outta her free will.
By proxy, Shidon crossed it too. Who is he? Well, he's Chekhov's Gunman Shinma that gave her and Miyu the trinkets that kept Miyu from sensing Chisato's growing Shinma energy. Double when he awakens Chisato's powers for the first time... and manipulates her into killing Hisae.
And the Shinma that manipulated Miho via preying on her insecurities and her gorgeous singing voice is not much better either.
There are MANY of those, actually. Aside of Aiko we have Aya, Mari and her sister Rima, Ruri and Mamoru from the manga; Miyahito, Ryoko, and the Shinma armor from the OAV; Miho from the TV series as well as Yukari and Hisae and Yui Shuugakuin from the Alternate UniverseVampire Princess.